Everyone knows that force is what breaks a rope. But some might not realize
that the force a rope experiences depends heavily on the properties of the rope
itself. Few aspects of caving cause as much confusion as so-called shock
loads. When cavers use this term they are usually talking about dynamic
loads resulting from stopping a fall. When scientists talk about shock loads
they are concerned with extremely rapid load application, such as that
encountered in ballistics studies—nothing like what cavers mean. To a
scientist, our loading conditions are simple physics, the stuff Newton worked
on. But this "simple" Newtonian physics has important implications
for rigging and vertical technique.

Dynamic loads arise from acceleration or deceleration of a mass. Weight is
merely a static load, the consequence of gravity pulling on a mass. The
relationship between the weight of an object and the force generated by its
acceleration is more subtle. A falling body accelerates because of gravity, but
this does not cause a dynamic or "shock" load. The dynamic load is
caused when the body stops falling; when it is decelerated by an applied force.
This force can come from a rope, or the ground. As the rope begins to store the
energy of the falling climber, the load increases. It then decreases, during
the rebound, down to a static load, the climber’s weight.

Unlike popular usage, engineers and scientists have specific meanings for
the commonly used terms. This is not merely jargon, because it prevents some of
the misunderstandings that can lead to bad conclusions—a valid concern for
cavers. A force is simply a load; a push or pull on an object. It can be
thought of as a muscular effort. Formally, work is the product of force
and distance; the distance through which the force acts. When a 180-lb climber
ascends ten feet up a rope, he does 1800 lb-ft of work against gravity.

Energy is the capacity to do work. The amount of energy something
possesses is exactly equal to the amount of work it can do. These concepts are
needed to understand the way a rope behaves during a fall.

When a force is applied to an object, it deflects. Most objects deflect
elastically, like a spring, up to a point. When the applied force is removed,
the deflection goes away. Energy is stored in the elastic deformation. Think of
stretching a Slinky toy. Beyond an elastic limit, plastic deformation, or
yielding occurs. Think of ruining a Slinky by stretching it too far.
Energy is consumed in the yielding process; the object doesn’t go back
to its original shape.

Figure 1: Rope characteristics represented graphically: The curve shows
the elongation of a 10-foot length of 10-mm caving rope under loads increasing
to the point of failure. The energy of a falling climber is merely a
consequence of his static weight and the distance fallen. With these rope
characteristics, the load resulting from deceleration can be determined
graphically.

Every object can be viewed as a spring. It has a spring rate, a
measure of the force required to deflect it an incremental amount. With some
objects and materials, the spring rate is not constant, such as a rubber band
that at same point suddenly gets very hard to stretch any farther. Steel does
the opposite; once plastic deformation begins to occur, small increases in load
have much larger effects on elongation. Most ropes have a fairly constant
spring rate. Spring rate is easily seen graphically. It is the slope of a graph
of force versus deflection (load vs. elongation).

Static rope is designed to have low stretch. Therefore, it has a high spring
rate, compared to dynamic rope. A force versus deflection curve for a 10-ft
length of typical 10-mm static caving rope is shown in Figure 1. It is
important to note that this curve, and the spring rate derived from it, are for
a specific length of rope. As with a coil spring, increasing the length
decreases the stiffness, thus decreasing the spring rate. Cavers notice a lot
of bounce at the bottom of a long, free pitch. Nearer the top, the effective
"spring" length is short and the spring rate is higher; you don’t
bounce as much.

An interesting aspect of the curve in Figure 1 is that since work (or
energy) is the product of force and distance, the area under the curve equals
the energy required to break the rope.

In rock climbing, a belayed fall is a common occurrence. In a belayed fall
much of the energy of the falling climber is converted to heat, because of rope
drag through the carabiners at each point of protection. Because of rope
stretch between the belayer and the climber, this is true even if no rope
slippage occurs at the belay point (a "static" belay as viewed by the
belayer). But usually, a lot of additional energy is absorbed or converted to
heat by the belayer (a "dynamic" belay). These factors significantly
reduce the resulting loads. In caving, falls are rare, but often result in a
perfectly static belay, as is the case when a rebelay or secondary anchor
fails. Even if climbing rope were used for underground rigging, the loads
resulting from anchor failure would be much greater than the loads resulting
from a fall of the same length in a normal rock climbing situation.

Consider a caver, who falls from his anchor point, with some amount of slack
in the rope. From the definition of energy we see that he possesses energy (of
falling: kinetic energy) equal to the product of his weight and the distance
fallen. In this situation friction is negligible; his energy must be absorbed
by the rope. If his energy is greater than the rope’s energy storage capacity
(the area under the curve), the rope will break.

If his energy is less than the energy storage capacity of the rope, he will
experience an impact force, a dynamic load, when the slack goes out of the
rope. His impact force (the force he and the rope and all the other rigging
components experience) can be determined by starting to shade in (from left to
right) the area under the rope’s load-elongation curve. When the area shaded
equals the area represented by the falling climber’s energy, we can read the
impact force (cavers’ "shock load") right off the graph. To be
accurate the climber’s weight must be added to the resulting load, since it
exists independent of the fall and dynamic load. For simplicity, we’ll deal
only with the dynamic portion of the load.

In the example of Figure 1, a 180-lb person takes a 5-ft fall, yielding 900
lb-ft of energy. From the graph we see that this energy results in a load of
about 1900 lbs and a rope elongation of about 1 ft. This exercise shows,
without the use of equations, that the dynamic loads on ropes and belay anchors
are the consequence of rope characteristics.

Figure 2 shows another version of the load-elongation curve. This time the
elongation axis of the graph has been normalized to express elongation
as a fraction (of the original, unstretched length) for a variety of ropes,
etc. One foot of elongation in a 10-ft rope results from the same force that
will cause 2 ft of elongation in a 20-ft rope; on this graph each elongation
would be 0.1.

Now, remembering the definition of energy, we’ll note that doubling the
amount of slack in the rope before falling will double the distance fallen
before the slack is used up, thus doubling the kinetic energy.

Combining these concepts lets us conclude that any increase in energy caused
by increasing the amount of rope slack will correspondingly increase
elongation, leaving the percent elongation unchanged. No change in percent
elongation means no change in load; and the concept of fall-factor is
born.

Fall-factor is the ratio of distance fallen to available rope slack (Figure
3). From this simple physics exercise we conclude that all falls of the same
fall factor result in the same deceleration rate, regardless of the absolute
distance fallen. In practical caving applications, conditions such as knot
tightening, deflection of the human body, and harness movement on the body tend
to significantly reduce loads in short-fall and low-fall-factor situations. But
for our purposes we’ll use this conservative and simplified model of reality.
The climber in the examples above who fell from his belay point had a fall
factor of one, regardless of the amount of slack.

In Figure 2 the energy storing capabilities of the ropes, webbing and chain
can be used to determine the load (force) experienced by a climber falling with
various fall factors.

Figure 2: Comparison of Load-Elongation Characteristics of several rope
types: here elongation is expressed as a fraction of unstretched rope length:
the curve is valid for any length of rope. The calculations show the
determination of fall factor for a reasonable threshold of survivability, the
UIAA standard. Rope curves are composites based on manufacturer data and our
own testing. Webbing curve is based on Brew (1977).

Figure 3: Fall Factors illustrated

For the chain the total area under the curve, to the point of failure, is
about 120 lb-ft per foot of rope/chain. That represents the energy, per
foot of unstretched chain, that will result in enough load to break the chain.
Fall factor, the total number of feet you can fall, per foot of rope, equals
the area under the curve divided by the climber’s weight.

In the case of the chain, a 180 lb-climber would break the chain with a fall
of fall factor 0.67 (120 lb-ft per foot divided by 180 lbs. equals 0.67).

Unfortunately, humans cannot survive deceleration forces of 7800 lbs, the
strength of the chain. At best, they can take about 15 G’s (a force of 15 times
their weight). The Union Internationale des Associations d’Alpinisme (UIAA)
uses 2650 lbs as a maximum tolerable load for evaluating climbing ropes. For
2650 lbs on the chain, we calculate a fall factor of about 0.07. Helpful hint:
do not belay with chains.

From the climbing rope curve, we calculate that a fall factor of 2.3 (by
definition, fall factor 2 is the maximum possible) would be needed for a 180-lb
climber to experience a deceleration load of 2650 lbs. In other words a fall of
any length on a climbing rope might be survived if the faller didn’t hit the
ground or a ledge before being decelerated by the rope. Hence the survival rate
of bungee jumpers.

From the curve for a 10-mm caving rope, we calculate that 2650 lbs equates
to a fall factor of 0.7 for a 180-lb climber. This confirms what rope
manufacturers tell us; lead climbing with caving rope can be risky and painful.
A fall on a 10-mm caving rope will end in a load about three times greater than
that resulting from the same fall on a climbing rope. A stronger 11.5-mm caving
rope would result in a load about 10% higher yet, because its spring
rate is 10% higher.

The issue of rope softening

Readers should be aware that the curves in Figures 1 and 2 are roughly
accurate for new ropes. A lot of attention has been given to the reduction in
rope strength that occurs with age. However, very little has been done to
assess the change in spring rate, and the energy handling capability of an old
rope. Unfortunately, spring rate is in most ways much more important for
surviving falls than the strength value. Tests by Smikmator (1986) and Kipp
(1979) clearly show that old rope is stiffer and produces higher loads than a
new rope subject to the same fall. Testing by Stibranyi (1986) on
Czechoslovakian climbing ropes produced the opposite results. Theory would tend
to support the former conclusions, though. Testing by the German Alpine Club
(Microys, 1977) showed a significant increase in stiffness of new climbing
ropes that were cold and wet.

Tests conducted in a study by Smith (1988) indicate that treatment with
concentrated fabric softener reduced the strength of a new rope. Frank (1989)
showed that certain ropes treated with dilute softener (per manufacturer’s
recommendations) were stronger than the same rope without softening, after
aging and washing. Frank reported that the likely mechanism at work explaining
these results is that the fiber lubricants contained in new rope are lost with
age, allowing the fibers to cut one another. Fabric softener replaces some of
the lubricants. Excess softening leaves the rope effectively wet, with the
corresponding loss in strength.

With this mechanism in mind, a further argument for treatment with fabric
softener would be its effect on spring rate. Since a rope’s spring rate is
determined by both nylon material properties and fiber weave, it is likely that
fabric softener will help prevent stiffening due to loss of internal
lubrication. In dynamic situations, the underlying physics shows that
preserving the spring rate is as important as preserving its strength toward
the goal of avoiding rope breakage. Probably more importantly, preserving the
spring rate will avoid the higher climber loads in falls that would come from a
rope that had become stiffer with age.

Strength misconceptions

Now that everyone understands the science, let’s go back and look at the
popular misconceptions. Much of the caving literature uses such terms as
"shock strength." This probably originated from intuitive physics,
which while frequently accurate, has failed us miserably here.

The fallacy probably came from a situation like this. Two ropes, one nylon
and one polyester, had the same strength, as measured by pull testing. But in a
fall-factor = 1 situation, the polyester snapped like a twig and the nylon was
unharmed. The obvious conclusions are that "shock strength" is a
radically different property than tensile strength, that dynamic testing is
needed to evaluate it, and that polyester has lower "shock strength"
than nylon.

To an extent the latter conclusion serves us; we avoid belaying with
polyester rope. But the first two conclusions are dead wrong, and have
encouraged us to do a lot of unscientific dynamic testing, sometimes drawing
even more inaccurate conclusions. Misinterpretation of test results has led
cavers and rescue enthusiasts to believe that the speed of load application
encountered in falls greatly affects the strength of the rope. Testing of nylon
materials and seat belts by the aircraft industry simply does not support this
(Figucia 1969). As we have seen, the speed of load application affects the
value of the load (because of the deceleration rate), not the strength
of the rope.

Probably the most harmful result of this situation is that "shock
strength" has encouraged us (and equipment manufacturers, through
competition) to make everything stronger. Some have advocated making caving
rope stronger, to resist being cut by ascenders in the event of an anchor
failure. Unfortunately, if low stretch is to be retained, stronger static rope
will necessarily be stiffer, and the benefits of higher strength may not be
realized because the greater stiffness causes higher deceleration loads. The
probable end result is that there is a negligible reduction in chance of rope
failure and a significant increase in chance of victim injury from high
deceleration loads.

Figure 4: The Figure 9 Knot

Another concern of dynamic loading is knot strength. Sharp bends in knots
result in some fibers being loaded much more than others—a stress
concentration occurs. Consequently they reach their ultimate load
capability first and fail, leaving the remaining fibers more highly loaded and
subject to failure. Knots used in caving reduce the rope’s strength by 30-60%.
For normal loading this is completely irrelevant. It only becomes important
when the knotted rope strength falls below the threshold of human load
tolerance in a dynamic situation such as a fall. Standard water knots,
bowlines, lasso-bowlines, and figure-8s will reduce strength by about one half.
A knotted 8mm rope, then, can fail with loads less than 2000 lbs. Thus knots
with more gradual bends, causing less stress concentration, such as the
figure-9 knot (Figure 4), must be used. It is reported to cause a strength loss
of about 30% (Marbach 1980).

In some circumstances, knot slippage can reduce dynamic loads by decreasing
deceleration rates. Use of shock-absorber knots has been advocated by several
caving texts. Alan Warild (1989) presents convincing evidence that the effect
is too variable and unreliable to be used in caving. The shock-absorbing knots,
usually overhand loops, might reduce loads, but they will definitely
reduce strength of the whole system, provided the rest of the rigging is done
correctly.

Recommendations

In summary, while an investigation into dynamic loading might initially
appear to be merely an academic pursuit, several specific recommendations arise
from it. In addition, the general concept of "stronger means better"
is further revealed as dangerous and nonproductive. Some important implications
of this study are:

Absolute fall height is not nearly as
important as fall factor.

Don’t belay lead-climbers with caving
rope.

Avoid fall factors much greater than 0
with caving rope

Rig rebelays with the shortest rope
loop possible.

Rebelays close to the top anchor must
be bombproof, because there is so little rope to absorb energy.

Position primary and secondary anchors
at pitch heads to minimize fall factor in event of anchor failure.

Treat ropes with fabric softener
suitable for nylon fabric. Follow label directions and do not use high
concentrations.

Acknowledgments

We would like to thank Dr. Walter Paul for his comments and observations on
the use of ropes in caving and climbing. Dr. Paul is a nylon rope specialist at
Martin-Marietta and an advisor to the US Navy on rope use. Dr. Peter Gibbs of
Gibbs Products shared his observations on the popular misconceptions of rope
physics and their implications. We would also like to thank Steve Hudson of
PMI, and fellow STC members Andy Grubbs and Steve Worthington for their
assistance in reviewing this article.

Ó 1990-1998 by William K. Storage
and John H. Ganter. All Rights Reserved. No part of this document may be
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Disclaimers: This document is an account of independent research, and
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